Ford Mustang Cylinder Head Porting - Head Trip

Cylinder-Head Porting: A Key To Power Production

Jim Smart

November 29, 2006

Cylinder-head port design and modification has been a source of speculation for as long as there have been internal combustion engines. Because engines are power-making air pumps, airflow volume and velocity through the ports and chambers is significant to making power. But did you know there's a lot more to cylinder-head porting and power than cylinder heads themselves?

Contrary to what you might have learned in bench racing, larger port size does not always mean more power. Port shape is the key to power and when that power becomes available. Cylinder-head porting begins with the proper cylinder head, intake manifold, and carburetor. It also begins with proper header selection because what you bolt to a cylinder head becomes the strongest or weakest link in the power equation. All the gains of the port job are lost at the intake manifold or exhaust headers if we don't make the right choice.

Porting begins with the intent for the engine. What do you want your engine to do? Are you going racing? If so, what kind of racing? If you're building for the daily commute, go more for low-end torque than high-end power. Perhaps the commute will involve some weekend racing or friendly open road rivalry. That means the engine must do both effectively.

Preparation Before PortingPorting begins with the carburetor, intake manifold, and header. It must first begin with induction and exhaust because a port job is only as productive as its weakest link. Carburetor sizing and intake-manifold port matching are crucial to the success or failure of a port job.

Carburetor sizing is critical to performance with or without porting. Too much carburetor can be as bad as not enough. Below is a quick-reference formula to follow for carburetor selection. Each application may call for a little more or a little less depending on your expectations and engine modifications.

Proper carburetor sizing is only the beginning of proper porting. Intake manifold selection is the next phase. Manifold selection isn't just about single-plane versus dual-plane, it's also about port compatibility. Intake-manifold ports must match cylinder-head ports. Any size difference creates all kinds of airflow disturbance issues that adversely affect power. Manifold and cylinder-head port sizes are rarely spot on, even when made by the same manufacturer. Blindly installing an intake manifold without checking port compatibility is just plain foolish. Your engine will run and it will make power, however, it won't make as much power as it would with matched ports.

Porting TerminologyShort-side turn: The short-turn wall from the port floor to the valve seat.

Porting BoothCylinder-head porting isn't just about increasing volume; it's about improving flow quality at the same time. Volume always helps power, but we want smooth volume. Rises and bumps in the port walls, roofs, and floors cause irregularities (turbulence) in airflow.

Intake ports manage flow differently than exhaust ports. On the intake side, we want to increase air speed and volume. On the exhaust side, we want to limit restrictions that impede exhaust gas flow and retain heat. We want hot gasses to roar out of the chamber rapidly and with a minimum of turns. This rapid flow of hot gasses helps on the intake side during valve overlap. As hot gasses roar out, cool air and fuel are drawn in. This is bonus energy we wouldn't have without good valve overlap.

Another consideration for your porting efforts is valve lift. You can flow all the air in the world on a flow bench, but if valves aren't sized, ground, and faced properly, all that effort will be lost to valve interference. Airflow varies depending on the distance between the valve and its seat. What's more, valve seat angle(s) determines flow as well. Sometimes, we get better airflow with less valve lift. Air is going to behave differently as the valve face moves farther away from the seat.

Another important consideration is valve seat and valve shape. A five-angle valve job will deliver better airflow than a three-angle because the path becomes smoother across the seat and valve.

Going with larger valves doesn't always help power. Again, how do you intend to use the engine? Good low-end torque calls for conservative valve sizing depending on how you're going to use the engine. For velocity and greater volume at high revs, larger valve sizing becomes important.

There's a lot of debate about what to do with combustion-chamber surfaces. To take advantage of valve sizing, we have to watch valve shrouding in the chamber. Larger valves have little worth when there's lots of iron or aluminum in the way. Air and hot gasses need to flow smoothly around open valves. Shrouding is affected by chamber shape as well as how far the valve opens. If you're running a lot of lift and the valve head clears, valve shrouding becomes less of an issue.

Chamber mods go well beyond valve shrouding. Rough surfaces affect fuel atomization and ignition. They can also cause detonation by way of hot spots that glow under hard acceleration. Hot spots can cause fuel to ignite prematurely, adversely affecting performance and engine life.

Each trip to the porting booth should include repeat journeys to the flow bench to chart progress, enabling you to see what porting has done for each head. Although it demonstrates what works and what doesn't, a flow bench doesn't tell all. Eyes and ears need to tell the rest. Examine each port carefully and watch for the irregularities mentioned elsewhere in this article.

Displacement

Suggested Carburetor Sizing

250-320ci

500 cfm Mild 600 cfm if fitted with a high-performance camshaft and heads

Photo Gallery

Affordable Port WorkAffordable factory head porting is available from PowerHeads in Southern California. PowerHeads uses CNC technology to hog out the intake and exhaust ports on 260, 289, 302, and 351W heads to make them one of the greatest performance values in the industry. For under $1,000, PowerHeads takes your cylinder-head castings and after CNC-port work along with hand-massaging finish work, vastly improves performance. PowerHeads will install your valve springs at the same time. You can provide the head cores or PowerHeads can provide them. The choice is yours. If you want your cylinder heads ported and returned to you, please advise PowerHeads and document your request. If you don't, expect a different set of heads.

Wild, Weird ScienceThe science of port work is little more than common sense. Think of the flow of air and fuel as water. Water ripples and aerates as it cruises over surfaces. So do air and fuel droplets. Our objectives are smooth surfaces and turns. We want increased velocity while we're at it. Velocity is simply air speed, how fast air and fuel move through an intake port. On the exhaust side, we want hot gasses to leave quickly without a traffic jam. We actually want those hot gasses to help with intake flow when we have valve overlap.

Matchmaker, MatchmakerIntake manifolds and carburetors are just as significant to porting as the human eye and a grinder. Carburetors must match intake manifolds. Intake manifolds must match cylinder heads. When perfectly matched, airflow becomes smooth and plentiful. We cannot overemphasize proper port matching.